This invention relates to electrical contactor devices for controlling the application of electrical current to a load.
Semiconductor devices have been used in parallel with electrical contacts to reduce or suppress arcing during contact closure and opening, as shown in U.S. Pat. Nos. 3,474,293; 3,543,047; 3,555,353; 3,558,910; 3,639,805; and 3,868,549, and an article entitled "Commutating Relay Combined Solid State Switching" by J. S. von Brimer, April 1965, 13th Annual National Relay Conference.
In many of these prior art devices, the semiconductor devices are connected in parallel with the contactor elements, and any leakage current through the semiconductor device would be felt at the output terminals of the contactor. Also, it is possible that transient voltages might trigger the semiconductor devices to the conducting state inadvertently and the load would thereby be connected to the source through the semiconductor although the contacts were not closed. This condition is not only undesirable since this could permanently damage the semiconductor, but it might also be dangerous, especially if maintenance personnel were in physical contact with the output of the contactor.
Electrical contactors manufactured to NEMA specifications are required to handle voltages up to 600 volts RMS, yet mass produced semiconductor devices, such as triacs and silicon controlled rectifiers, generally have a maximum voltage rating in the order of 500 volts. While some semiconductors having a higher voltage rating are available, they can only be obtained at a significant increase in cost.